Now very active efforts are made for development of (the Intelligent Transport System: ITS) to be put into practical use in the 21-st century. The ITS is a transport system constructed by using the most advanced data communication technology in which roads and vehicles are integrated with each other. Purposes of construction of this system includes improvement of safety and transport efficiency, realization of improved comfort, and contribution to better environmental conditions.
The most advanced technology used for construction of the ITS is (the Automated Highway System: AHS). The AHS enables completely automatic mode of driving by means of automated controls over information transaction between a road and a car as well as over the car, which makes it possible to reduce work load to a driver and to improve safety and smoothness in driving.
The AHS is realized by the "road-to-vehicle communications" executed between information infrastructure on a road and vehicles and the "vehicle-to-vehicle communications" executed between vehicles. The AHS can respond to various situations by controlling running of vehicles by means of bilateral communications between a road and vehicles as well as of bilateral communications between vehicles.
However, it can be anticipated that the following problems would occur in relation to introduction of automatic run of vehicles in the future.
It is anticipated that the AHS will be managed by means of driving controls by way of mobile communications based on zone configuration for effective utilization of channels. In that case, if a traffic jam occurs on a road, a number of vehicles in one radio zone substantially increases. A number of frequency bands available for the AHS is limited, however, so that shortage of communication channels occurs.
In a case of a portable telephone based on the conventional technology, when the situation as described above occurs, the busy state is notified to a user, and the user is required to stand by for a while, but in a case of AHS, real time controls are provided over vehicles actually running, so that generation of the busy state must be prevented by any means. To achieve the object described above, a system for continuously providing control information to vehicles requiring services under limited environmental conditions is required.
Further, explosive increase of communication equipment put in actual use may require a far larger number of channels than an expected one for a period of time in the future. There is the strong needs for development of a method of suppressing the overflow as described above.